This invention relates to a method of despreading a GPS spread spectrum signal received at a mobile unit with assistance from a base station.
It is well known to provide a GPS receiver in which replica GPS satellite pseudorandom noise (PRN) code signals are continuous generated and correlated with received GPS signals in order to acquire them. Typically, as the replica codes are likely to have a different code phase to those of the received GPS signals and also a different frequency due to Doppler shift between the receiver and orbiting satellites, a two dimensional code frequency/phase sweep is employed whereby such a sweep will eventually result in the incoming PRN code having the same frequency and code phase as that of the locally generated replica. If detected, the code is acquired and tracked, and the pseudorange information may be retrieved from which the position of the receiver may be calculated using conventional navigation algorithms.
It is further known to provide a mobile cellular telephone incorporating such a GPS receiver for the purpose of enabling operators of cellular telephone networks to determine the location from which a call is made and, in particular, for an emergency call to the emergency services. Of course for an emergency call, it is desirable for the call location to be available as soon as possible, however, from a xe2x80x9ccold startxe2x80x9d where the GPS receiver does not have access to up to date ephemeris data or even worse from a xe2x80x9cfactory cold startxe2x80x9d where the GPS receiver does not have an up to date almanac, the time to first fix (TTFF) can be anywhere between 30 seconds and 5 minutes.
In order to reduce the TTFF, a GPS receiver may be provided with base station assistance in order to acquire GPS signals more quickly. Such assistance may include the provision by the base station to the receiver of a precision carrier frequency reference signal for calibrating the local oscillator used in the GPS receiver and, as obtained by the base station, the data message for up to date satellite almanac and ephemeris data from which Doppler shift for satellites in view can be determined together with the current PRN code phase. With such assistance, it is possible to sweep only a narrowed range of frequencies and code phases in which the GPS PRN code is known to occupy, thereby reducing the number of code instances that need to be checked and thus reducing the time for code acquisition. Base station assistance is further described in U.S. Pat. Nos. 5,841,396 and 5,874,914 which are incorporated herein by reference.
It is an object of the present invention to provide an improved method of despreading a GPS spread spectrum signal received at a mobile unit with assistance from a base station with which the TTFF may be reduced.
According to the present invention, there is provided such a method comprising the steps of:
(a) receiving and despreading GPS signals at the base station and deriving GPS signal information therefrom, ideally including Doppler and code phase information;
(b) estimating the location of the mobile unit;
(c) modifying the GPS signal information so as reflect the GPS signal characteristics as would be observed at the estimated location of the mobile unit;
(d) transmitting the GPS signal information from the base station to the mobile unit;
(e) generating replica signals containing PRN codes corresponding to those of the GPS signal using the modified GPS signal information; and
(f) performing a correlation of the GPS signal and the replica signals in order to acquire the GPS signal.
The method of the present invention enables a narrower range of frequencies and/or code phases in which the GPS PRN code is known to occupy to be identified. As such, fewer code instances need to be checked and the time for GPS signal acquisition and thus the TTFF may be reduced.
Step (c) may be done at the base station in which case the modified GPS signal information is transmitted from the base station to the mobile unit, i.e. step (c) is done before step (d). Where this is so, whilst it may be convenient for the base station to transmit the modified GPS signal information to the mobile unit using direct RF communication, transmitting the modified GPS signal information from the base station to the mobile unit is also intended to include indirect transmission such as via land lines or using repeater stations. Furthermore, the initial estimate of the location of the mobile unit may be determined in the mobile unit and transmitted to the base station.
Also, provided in accordance with the present invention the combination of a base station and a mobile unit for despreading a GPS spread spectrum signal received at a mobile unit with assistance from a base station; wherein the base station comprises a GPS receiver for receiving and despreading GPS signals at the base station and deriving GPS signal information therefrom, a processor arranged to modify the GPS signal information so as reflect the GPS signal characteristics as would be observed at an estimated location of the mobile unit; and a transmitter for transmitting the modified GPS signal information from the base station to the mobile unit; and wherein the mobile unit comprising a GPS receiver for receiving the GPS signal, a communications receiver for receiving GPS signal information from a base station; and a processor arranged to generate replica signals containing PRN codes corresponding to those of the GPS signal using the modified GPS signal information and perform a correlation of the GPS signal and the replica signals in order to acquire the GPS signal. In addition, a further combination is provided wherein the base station comprises a GPS receiver for receiving and despreading GPS signals at the base station and deriving GPS signal information therefrom, and a transmitter for transmitting the modified GPS signal information from the base station to the mobile unit; and wherein the mobile unit comprising a GPS receiver for receiving the GPS signal; a communications receiver for receiving GPS signal information from a base station; and a processor arranged to modify the GPS signal information so as reflect the GPS signal characteristics as would be observed at an estimated location of the mobile unit, generate replica signals containing PRN codes corresponding to those of the GPS signal using the modified GPS signal information and perform a correlation of the GPS signal and the replica signals in order to acquire the GPS signal.
Further provided in accordance with the present invention is a base station for providing such assistance comprising a GPS receiver for receiving and despreading GPS signals at the base station and deriving GPS signal information therefrom, a processor for modifying the GPS signal information so as reflect the GPS signal characteristics as would be observed at an estimated location of a mobile unit; and a transmitter for transmitting the modified GPS signal information from the base station to the mobile unit.
Yet further provided in accordance with the present invention is a mobile unit comprising a GPS receiver for receiving the GPS signal; a communications receiver for receiving GPS signal information from a base station; and a processor arranged to modify the GPS signal information so as reflect the GPS signal characteristics as would be observed at an estimated location of the mobile unit, generate replica signals containing PRN codes corresponding to those of the GPS signal using the modified GPS signal information and perform a correlation of the GPS signal and the replica signals in order to acquire the GPS signal